The index profile of optical fibers is typically described as the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber cladding is shown on the y-axis. Thus the terms “step,” “trapezoid,” “alpha,” or “triangle” index profile are used to describe graphs having curves with the shapes of a step, trapezoid, alpha, or triangle. These curves are generally representative of the theoretical or set profile of the optical fiber. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
An optical fiber conventionally includes an optical fiber core which transmits and/or amplifies an optical signal, and an optical cladding which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the cladding ng (i.e., nc>ng). The propagation of an optical signal in a single-mode optical fiber is broken down into (i) a fundamental mode in the core (called LP01) and (ii) secondary modes guided over a certain distance in the core-cladding assembly, called cladding modes.
Conventionally, step-index fibers, also called single-mode fibers (SMFs), are used as line fibers for optical fiber transmission systems. These fibers exhibit chromatic dispersion and a chromatic dispersion slope meeting specific telecommunications standards as well as standardized cut-off wavelength and effective area values.
To facilitate compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) defined a standard reference ITU-T G.652, with which a standard optical transmission fiber (i.e., a standard single mode fiber or SSMF) should comply. The ITU-T G.652 standard and each of its attributes (i.e., the A, B, C, and D sub-standards) are hereby incorporated by reference.
For a transmission fiber, the ITU-T G.652 standard recommends: (i) at a wavelength of 1310 nanometers, a mode field diameter (MFD, also herein referred to as “2W02”) of between about 8.6 and 9.5 microns with a tolerance of ±0.6 microns (i.e., a MFD of between about 8.0 microns and 10.1 microns); (ii) a cable cut-off wavelength of 1260 nanometers or less; (iii) a zero dispersion wavelength (ZDW) (i.e., the chromatic dispersion coefficient) of between about 1300 and 1324 nanometers; and (iv) a chromatic dispersion slope (i.e., a zero dispersion slope or ZDS) of 0.092 ps/(nm2·km) or less at the zero dispersion wavelength. Conventionally, the cable cut-off wavelength (λCC) is measured as the wavelength at which the optical signal is no longer single mode after propagation over twenty-two meters of optical fiber, such as defined by subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard, which is hereby incorporated by reference.
Moreover, for applications including fibers intended for optical systems laid to private homes (e.g., fiber to the home (FTTH) or fiber to the curb (FTTC)), reducing bending losses is important, particularly when the optical fiber will be clipped or coiled in a miniaturized optical box. Standards have thus been defined to impose bending-loss limits on fibers intended for such applications.
The ITU-T G.657B standard reproduces many of the requirements of the ITU-T G.652 standard while imposing stricter limits on bending losses. That said, the ITU-T G.657B standard provides a more generous range for mode field diameter (i.e., mode field diameter (2W02) between 6.3 microns and 9.5 microns, with a tolerance of +/−0.4 micron).
In this regard, the ITU-T G.657B standard suggests that at a wavelength of 1550 nanometers, the bending losses should be less than 0.003 dB/turn for a radius of curvature of 15 millimeters (i.e., 0.03 dB for ten turns), 0.1 dB/turn for a radius of curvature of 10 millimeters, and 0.5 dB/turn for a radius of curvature of 7.5 millimeters. Furthermore, at a wavelength of 1625 nanometers, the bending losses should be less than 0.01 dB/turn for a radius of curvature of 15 millimeters (i.e., 0.1 dB for ten turns), 0.2 dB/turn for a radius of curvature of 10 millimeters, and 1 dB/turn for a radius of curvature of 7.5 millimeters. The ITU-T G.657 standard and each of its attributes are hereby incorporated by reference.
For a given optical fiber, a value known as the MAC value is defined as the ratio of the mode field diameter (2W02) of the optical fiber at 1550 nanometers to the effective cut-off wavelength (λCeff). Conventionally, the effective cut-off wavelength is measured as the lowest wavelength at which the optical signal is single mode after propagation over two meters of optical fiber, as defined by subcommittee 86A of the IEC in the IEC 60793-1-44 standard, which is hereby incorporated by reference. The MAC value can be used to assess the performance of the optical fiber.
Commonly assigned European Patent No. 1,845,399 (and its counterpart U.S. Pat. No. 7,587,111) and commonly assigned European Patent No. 1,785,754 (and its counterpart U.S. Pat. No. 7,623,747), which are hereby incorporated by reference, disclose experimental results and establish a relationship between the MAC at a wavelength of 1550 nanometers and bending losses at a wavelength of 1625 nanometers with a radius of curvature (i.e., a bend radius) of 15 millimeters in a standard SSMF step-index fiber. In particular, these documents illustrate that the MAC value has an effect on the bending losses of the optical fiber and that these bending losses can be reduced by reducing the MAC. Reducing the MAC, however, can result in noncompliance with the ITU-T G.652 standard.
In this regard, reducing bending losses while retaining certain optical transmission parameters (e.g., mode field diameter and cut-off wavelength) constitutes a major challenge for FTTH or FTTC applications.
U.S. Pat. No. 7,164,835 and U.S. Patent Publication No. 2007/0147756, which are hereby incorporated by reference, describe fiber profiles having limited bending losses. These fiber profiles, however, only barely comply with the criteria of the ITU-T G.652 standard, particularly with respect to mode field diameter and chromatic dispersion.
Holey fibers are optical fibers having a regular arrangement of air holes running along their length to act as a part of a cladding.
Moreover, holey-fiber technology makes it possible to achieve improved bending-loss performance (i.e., reduced bending losses). In this regard, optical fibers implementing this technology have been proposed.
For example, U.S. Pat. No. 6,901,197, which is hereby incorporated by reference, describes an optical fiber having a central core and an optical cladding. A plurality of holes is formed in the optical cladding. These holes are arranged to form concentric hexagons.
U.S. Patent Publication No. 2006/0024009, which is hereby incorporated by reference, describes a single mode fiber including a central core and an optical cladding. The optical cladding includes a plurality of cylindrical air holes which form a network. The air holes are arranged periodically such that the center-to-center distance between two adjacent air holes is at least equal to 1.5 times the wavelength of the light propagating in the optical fiber.
U.S. Pat. No. 6,636,677, which is hereby incorporated by reference, describes an optical fiber including a central core and an optical cladding in which a plurality of air holes arranged in concentric circles is formed.
U.S. Pat. No. 5,907,652, which is hereby incorporated by reference, discloses a multimode fiber having a central core, a multimode intermediate optical cladding, a first and a second external optical cladding, and a polymer coating. According to this document, air holes are formed in the first outer cladding. The air holes occupy a volume greater than 75 percent of the volume of the first outer optical cladding.
U.S. Patent Publication No. 2006/0045448, which is hereby incorporated by reference, describes an optical fiber having a central core and an optical cladding in which a plurality of cylindrical air holes are arranged in a ring.
Another optical fiber including a central core and an optical cladding having cylindrical air holes distributed in a ring is described in the article “Hole-assisted fiber design for small bending and splice losses,” from IEEE photonics technology letters, vol. 15, No. 12, December 2003, which is hereby incorporated by reference. The diameter of the holes is equal to the diameter of the central core.
The article “High performance optical fibers for next generation transmission systems,” from Hitachi Cable Review, No. 22, August 2003, which is hereby incorporated by reference, also describes an optical fiber including a central core and an optical cladding having six air holes. This article, however, does not give any details concerning the dimensions of the different elements of the optical fiber.
Similarly, the article “Field installable connector optimized for holey fiber,” by Y. Kato, K. Suzuki and K. Ohsono from the proceedings of the Optical fiber communications conference, OFC 2007, communication NthA2, which is hereby incorporated by reference, also describes an optical fiber including a central core and an optical cladding having six air holes. This article, however, does not give details concerning the dimensions of the different elements of the optical fiber.
Finally, the article “A novel fabrication method of versatile holey fibers with low bending loss and their optical characteristics,” by G. H. Kim, Y. G. Han, H. S. Cho, S. H. Kim, S. B. Lee, K. S. Lee, C. H. Jeong, C. H. Oh, H. J. Kang, from the proceedings of the Optical fiber communications conference, OFC 2006, communication OWI2, which is hereby incorporated by reference, describes an optical fiber including a central core and an optical cladding in which six air holes are arranged in a ring. The diameter of the holes is greater than the diameter of the central core.
The previously discussed documents, however, fail to provide an optical fiber having low bend losses and a relatively high MAC value.
In this regard, there exists a need for an optical fiber having good resistance to bending losses, while still possessing a relatively high MAC value.